The effects of thin surface-applied oxide coating films on the selective oxidation of alloys

Abstract

The effects of thin surface-applied Al 2O 3-Y 2O 3 composite films on the selective oxidation of chromium in Ni-10Cr in air at 1000 °C have been studied. An integral chromia scale was established on specimens coated with an A1 2O 3-Y 2O 3 film during oxidation for 100 h and the weight gains were almost two orders of magnitude lower than those of uncoated specimens. Mechanisms to account for such effects have been explored. It is proposed that two oxygen transport processes influence the selective oxidation of an alloy: short-range diffusion of oxygen at the scale/alloy interface and long-range diffusion of oxygen in the bulk alloy. When the flux of oxygen due to shortrange diffusion at the scale/alloy interface is much greater than that due to long-range diffusion in the bulk alloy, a chromia scale is able to form at the scale/alloy interface, that is, selective oxidation of chromium is promoted. Otherwise, internal oxidation of chromium in this alloy occurs. It has been confirmed that increasing the number of sites for oxide to nucleate, by the application of a surface oxide film, accelerates short-range diffusion and reduces the critical content of chromium in the alloy required for the formation of a chromia scale. Such selective oxidation may be promoted by increasing the thickness of the film or by applying a film in which the transport of oxygen is much reduced. The beneficial effects of an Al 2O 3Y 2O 3 composite film on the selective oxidation of chromium in Ni-10Cr can be explained by this hypothesis.